1. Field of the Invention
This invention generally relates to an optical pick-up device for use in an optical disc system or the like, and, in particular, to a split type optical pick-up device for eraseably writing and reading information to and from an optical disc, which includes a stationary optical system and a movable optical system.
2. Description of the Prior Art
An optical pick-up device of an optical disc memory system is well known, and, in general, such a device includes an objective lens through which a laser beam is passed to form a small light spot on an optical information recording medium, such as an optical disc, thereby allowing to record and read or erase information to and from the recording medium. Such an optical disc system has a particularly large information storage capacity and an optical disc is removable.
In such an optical disc memory system, since an information pit, i.e., a unit of information to be recorded on an optical disc, is extremely small, in the order of 1 micron, in order to record and reproduce information to and from an optical disc accurately it is necessary to carry out focusing control, tracking control and seek control. Focusing control is typically carried out by displacing the objective lens along an optical axis thereof, and tracking control is carried out by displacing the objective lens in the tracking direction or in a direction transverse to a recording track of the optical disc. Seek control is typically carried out by moving the overall optical pick-up device first to a location in the vicinity of a target track by a coarse control operation and then to the exact desired location by a fine control operation.
However, as compared with the typical prior art pick-up device for a magnetic disc memory, an optical pick-up device weighs several hundreds grams in its entirety. For this reason, according to a control method in which the entire optical pick-up device is moved in the tracking direction, the inertia of the optical pick-up device becomes an important factor in the seek control operation for bringing a laser spot to a position in the vicinity of a desired track by moving the objective lens. Because of this, a high-speed seek operation is difficult to carry out and thus access time tends to be longer.
In an effort to cope with this problem, a split type or actuator seek type optical pick-up device has been proposed to realize high-speed accessing in an optical disc memory system. FIG. 5 schematically illustrates such a split type optical pick-up device. As shown in FIG. 5, such a split type optical pick-up device includes a movable optical system 2 which is located opposite a recording surface of an optical disc 1 which serves as an optical information recording medium. The movable system 2 executes a seek movement in the radial direction of the disc 1. Optically coupled to the movable optical system 2 is a stationary optical system 3 which is fixedly mounted on a mounting device or carriage.
The stationary optical system 3 includes a semiconductor laser 4. A laser beam emitted from the laser 4 passes through a coupling lens 5, whereby the laser beam is collimated. The laser light emitted from the semiconductor laser 4 has an elliptic light intensity distribution; however, when the laser light emitted from the laser 4 passes through an anamorphic prism 6, the laser light is shaped to have a circular light intensity distribution. Then, the laser beam is incident upon a beam splitter 7 where the laser beam is deflected toward a beam deflecting member 8 disposed within the movable optical system 2, so that the laser beam is deflected toward a bottom surface of the optical disc 1. In this instance, the laser beam passes through an objective lens 9 so that the laser beam is focused onto the bottom surface of the optical disc 1 in the form of a beam spot minute in size.
The optical disc 1 reflects this light impinging thereon, and this reflected light also passes through the objective lens 9 and then is deflected toward the stationary optical system 3 by the deflecting member 8. This returning light beam then passes through the beam splitter 7 and then enters into a light polarization beam splitter 12 after passing through a half wavelength plate 10 and a detector lens 11. The light beam is split into two beams by the light polarization beam splitter 12 and one of the split beams impinges upon a focus detecting light-receiving device 14 after passing through a cylindrical lens 13 while the other split beam impinges upon a track detecting light-receiving device 15. A tracking signal is detected by a well-known far field method and a focusing signal is detected by a known astigmatism method. And, an information signal is detected by a known differential method by taking a difference between these two signals detected by the two light-receiving devices 14 and 15.
In order to carry out a separate actuator driving operation, the light emitted from the stationary optical system 3 is collimated and enters into the movable optical system 2 in a radial direction of the optical disc 1. It is to be noted that although not shown specifically, the movable optical system 2 is also provided with various other elements, such as a driving device for driving the objective lens 9 to effect focusing and/or tracking.
Now, referring to FIGS. 6a and 6b, the principle of operation for detecting a tracking signal will be described. FIG. 6a illustrates the condition in which a laser light spot 16 condensed by the objective lens 9 is located at the center of a track 17 of the optical disc 1. In this case, since the light spot 16 is located at the center of the track 17, the laser beam is diffracted symmetrically with the track as a center of symmetry. As a result, a pair of light-receiving elements, photoelectric elements, 15a and 15b of the two-division type light-receiving device 15 for detecting a tracking signal, which is disposed inside the stationary optical system 3 receives diffracted light having a symmetrical light distribution pattern 18 as shown in FIG. 6a. As a result, outputs A and B from these light-receiving elements 15a and 15b, respectively, are equal in magnitude, i.e., A=B, which indicates the fact that the optical pick-up device is in a proper tracking position. If the location of the light spot 16 is shifted sideways from the central position of the track 17 as shown in FIG. 6b, the distribution pattern 18 of diffracted light becomes asymmetrical between right and left, so that there is produced a difference in magnitude between outputs A and B of the respective light-receiving elements 15a and 15b. A tracking signal is detected by taking an output difference A-B between the light-receiving elements 15a and 15b and a tracking control operation is carried out to reduce this difference between outputs A and B to zero.
In such a tracking control operation, in the case of a split type optical pick-up device, only the movable optical system 2, having a relatively small mass, is moved with respect to the optical disc 1, so that the energy for moving an object to effect tracking control could remain relatively low tracking could be carried out at high speed, e.g., access time within 100 msec. However, such a split type optical pick-up device is not free of drawbacks. In the first place, due to a deviation from the parallel condition between the carriage for movable system 2 and the collimated light projected from the stationary optical system 3, there could be an offset in the tracking signal. In addition, the movable optical system 2 may shift in position in the vertical direction during access operations due to a play in the carriage, deposition of debris and change in temperature, which could also cause an offset in the tracking signal.
When such an offset is produced in the tracking signal, the spot 16 is not located at the center, which could cause the tracking operation to be unstable. In addition, the information recording, reproducing and erasing characteristics could deteriorate. In particular, referring to FIGS. 7 and 8, when the deflecting member 8 is located at its initial position indicated by the solid line in FIG. 7, there is obtained a distribution pattern 18 of diffracted light on the tracking detecting light-receiving device 15 as indicated also by the solid line. Under these conditions, a detection signal obtained from the light-receiving device 15 can be set to zero offset by initial adjustment as shown in FIG. 8a. However, when the deflecting member 8 has been displaced over a distance d upwardly in the vertical direction as indicated by the phantom line in FIG. 7 due to movement of the movable optical system 2 for focusing control, the light path for the laser beam reflected from the optical disc 1 and advancing to the light-receiving device 15 within the stationary optical system 3 through the objective lens 9 and deflecting member 8 is also shifted in position upwardly over the corresponding distance d. As a result, the diffracted light distribution pattern 18 on the light-receiving device 15 is also shifted in position as indicated by the phantom line. Accordingly, there is produced an offset in a detection signal obtained from the light-receiving device 15 as compared with the initial condition as shown in FIG. 8b. With such a tracking detection signal having an offset, the laser light spot 16 will be located at a position away from the central position of the track 17 when a tracking control operation has been carried out, thereby producing an error having an amount delta as shown in FIG. 8b. Therefore, under such a condition, it may not be possible to carry out highly accurate recording and reproducing operation.
In order to prevent such an offset from occurring, it is necessary to eliminate plays in the assembly as much as possible by adopting a high assembling accuracy; however, such an approach pushes up the cost of manufacture. In another invention by the present inventor, a split optical pick-up unit has a movable optical system which contains a beam splitter serving as a light deflecting member. A tracking detecting device receives the light which has been reflected from the optical disc and has passed through the beam splitter to thereby detect a tracking signal without producing an offset. This related invention is disclosed in U.S. patent application Ser. No. 07/150,134 filed on Jan. 29, 1988, is entitled "A SPLIT TYPE OPTICAL PICK-UP DEVICE", and is assigned to the assignee of this application, and is incorporated in its entirety herein by reference. In this related invention, an information signal is detected by leading the light reflected by the beam splitter into the stationary optical system. In this case, however, the light reflected from the optical disc is split by the beam splitter into a transmitted light beam and a reflected light beam, for example, at a ratio of 1:9. As a result, a portion of the information signal is supplied to the tracking detecting light-receiving device, which does not use it, only to be wasted and thus the information signal to be supplied to an information signal detecting light-receiving device disposed within the stationary optical system is reduced by the corresponding amount. This affects the S/N ratio of information signal.